1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device, and in particular to a semiconductor device fabrication method capable of forming a highly accurate pattern by detecting a focal point at a time of exposure with high accuracy.
2. Description of the Background Art
In a conventional exposure apparatus, the focal point at the time of exposure is detected by guiding a detection light at a wide angle to a surface of a substrate constituting an object to be measured and thus increasing a reflectivity of the uppermost surface of the substrate. A light reflected from the substrate is detected by a light detector, and a position of the reflected light is measured thereby to detect the focal point at the time of exposure.
In recent years, a new exposure technique called the liquid immersion lithography has come to be closely watched. The liquid immersion lithography is a technique of making exposure with a liquid filled between a projection lens of a stepper and a wafer.
In the liquid immersion lithography, the liquid exists on a resist, and therefore it is difficult to detect the focal point at the time of exposure in real time in a method similar to the aforementioned case. In view of this, there has been proposed a method of detecting the focal point of the detection light in the air on another wafer stage and use the resultant data.
An exposure method using the conventional exposure apparatus is disclosed in Japanese Patent Laying-Open Nos. 09-036017, 2004-207709 and 2005-099648, and WO00/58761.
A non-polarized light is often employed as the detection light radiated on the substrate surface for detecting the focal point at the time of exposure. The detection light is radiated at a wide angle to increase the reflectivity of the detection light on the uppermost surface of the substrate as described above. Part of the detection light, however, enters films formed on the substrate and interferes by multiplex reflection in each thin film. As a result, the detection light becomes the reflected light covering a broad range of height (detection position) and width including the light reflected from the films lower than the uppermost surface of the substrate. For this reason, a position lower than the uppermost surface of the substrate may be detected on the one hand, and since the detection light is periodically changed due to the effect of multiplex reflection in the underlying thin films, a detection error may occur on the other hand. This problem tends to become conspicuous in the liquid immersion lithography. Nevertheless, a similar problem is encountered inherently in the normal exposure in the air.
As described above, the problem of the detection error of the focal point at the time of exposure is considered to become conspicuous for the liquid immersion lithography. The reason why this problem becomes conspicuous for the liquid immersion lithography is explained below.
In the liquid immersion lithography, the liquid exists on the resist at the time of exposure, and therefore the detection light for detecting the focal point passes through the liquid. In view of the fact that a refractive index of the light entering the liquid is larger than that of the light entering the air, however, the reflectivity of the detection light on the resist surface for the liquid immersion lithography is reduced as compared with the value for normal exposure.
Now, the relation between a difference of refractive indexes among a plurality of media and the reflectivity at a surface boundary between the media is explained.
According to the Fresnel formulae, the amplitude reflectivity, at a single surface boundary, of a light entering a second medium from a first medium at an angle of θ is given by Equations (1) and (2) described below respectively for an s-polarized light and a p-polarized light, for example. In Equations (1) and (2), n1 and n2 indicate refractive indexes of the first and second media, respectively. A light intensity reflectivity is expressed as a square of the amplitude reflectivity.
                    [                  Equation          ⁢                                          ⁢          1                ]                                                                      s          ⁢                      -                    ⁢          polarized          ⁢                                          ⁢          light          ⁢                                          ⁢                      r            s                          =                                                            n                1                            ⁢              cos              ⁢                                                          ⁢                              ϑ                1                                      -                                          n                2                            ⁢              cos              ⁢                                                          ⁢                              ϑ                2                                                                                        n                1                            ⁢              cos              ⁢                                                          ⁢                              ϑ                1                                      +                                          n                2                            ⁢              cos              ⁢                                                          ⁢                              ϑ                2                                                                        (        1        )                                          p          ⁢                      -                    ⁢          polarized          ⁢                                          ⁢          light          ⁢                                          ⁢                      r            s                          =                                                                              n                  1                                ⁢                                  /                                ⁢                cos                ⁢                                                                  ⁢                                  ϑ                  1                                            -                                                n                  2                                ⁢                                  /                                ⁢                cos                ⁢                                                                  ⁢                                  ϑ                  2                                                                                                      n                  1                                ⁢                                  /                                ⁢                cos                ⁢                                                                  ⁢                                  ϑ                  1                                            +                                                n                  2                                ⁢                                  /                                ⁢                cos                ⁢                                                                  ⁢                                  ϑ                  2                                                              =                                                                      n                  1                                ⁢                cos                ⁢                                                                  ⁢                                  ϑ                  2                                            -                                                n                  2                                ⁢                cos                ⁢                                                                  ⁢                                  ϑ                  1                                                                                                      n                  1                                ⁢                cos                ⁢                                                                  ⁢                                  ϑ                  2                                            +                                                n                  2                                ⁢                cos                ⁢                                                                  ⁢                                  ϑ                  1                                                                                        (        2        )            
Equations (1) and (2) show that the larger the difference between the refractive index (n1) of the first medium and the refractive index (n2) of the second medium is, the higher the reflectivity at the single surface boundary. In a case where the difference is reduced between the refractive index on an exposure medium constituted of a liquid and the refractive index on the resist as in the liquid immersion lithography, therefore, it becomes difficult to increase the reflectivity of the uppermost surface of the substrate (the surface boundary between the exposure medium and the resist, for example). As a result, a greater amount of detection light intrudes into the resist at the time of detecting the focal point, and the detection light interferes by multiplex reflection on internal reflection surfaces. As compared with the normal exposure, therefore, detection accuracy of the focal point is liable to be deteriorated.
Reduction in detection accuracy of the focal point at the time of exposure poses problems that the exposure accuracy is reduced and the high-accuracy pattern formation is hampered.